The durability of helicopter rotor blades is dependent to a large extent on the erosion of the component by friction or by impact of finely divided solid or liquid particles. There is no way to avoid this friction or particulate impact during use of these components and therefore some means is needed to protect the components against erosion.
For example, the air through which the helicopter rotor blade rotates may contain particulate matter, such as sand. The size of the sand particles typically ranges from about 0.1 to 2000 microns and more typically from about 20 to 30 microns in diameter. If the air contains sand, the sand impinges upon the rotor blades as they rotate, thereby causing abrasion to the blades, or at least to portions thereof. Unless the blades are adequately protected, such repetitive abrasive contact can eventually cause the blades to erode.
The potential for erosion also exists if the rotor blades circulate through air containing water droplets. The size of water droplets ranges from about 1000 to 4000 microns and is typically about 2000 microns in diameter. Although the size of the water droplets is typically greater than the size of sand, under high velocity conditions, water droplets may behave similar to sand, thereby causing erosion to the rotating rotor blades.
Moreover, the combination of rain and sand can exacerbate the amount of abrasion and/or erosion. As a result, when translating a component through air comprising both rain and sand, the potential for erosion further increases.
The potential for erosion is also a function of the force at which the particulate matter impacts the rotor blade. Specifically, as the impact force increases, so does the potential for erosion. The force at which the particulate matter impacts the rotor blade is dependent upon the geometric shapes of both the rotor blade and the impacting particle and their relative velocities. For example, the leading edge of a rotor blade is the portion of the blade that first cleaves through the air. Therefore, the leading edge is the portion of the blade most susceptible to erosion caused by the abrasive contact of particulate matter.
The amount of erosion to the rotor blade is also a function of the velocity at which the blade impacts the particulate matter or vice versa. In other words, the potential for erosion increases as the speed of the blade increases. For example, because a rotor blade typically rotates around a central axis, the velocity of the rotor blade, relative to the air, differs along the leading edge of the blade. More specifically, the velocity at a point on a blade is equal to the product of the distance from the center rotational axis and the rotational velocity. As the distance from the rotational axis along the leading edge increases, so does the rotational velocity. The outboard tip of the rotor blade is the furthest from the rotational axis. Therefore, the potential for erosion is greatest at the outboard tip of the leading edge of the rotor blade.
Various techniques have been attempted to minimize the amount of erosion to the leading edge of rotor blades. One technique includes adhesively bonding an appropriately shaped piece of ductile metal onto the leading edge of the blade, such that the ductile metal is an integral part of the blade. The ductile metal leading edge is typically constructed of nickel, which provides increased wear resistance. The extended exposure of the nickel to the impinging particulate matter, however, causes the ductile metal leading edge to erode. The eroded nickel must, therefore, be replaced. Because the ductile metal leading edge is adhesively bonded to the blade, replacing the ductile metal leading edge requires a certain amount of time and skill, which is not typically available in the field.
Repairs that are performed in the field are referred to as “field level” repairs because such repairs require an acceptable amount of time and a minimal amount of skill to complete. Repairs requiring an extended amount of time and a heightened skill level occur back at the aircraft depot and are referred to as “depot” repairs. Depot repairs are undesirable because depot repairs increase the amount of time that the aircraft is unavailable in comparison to a field level repair. Because the replacement of the ductile metal leading edge is considered a depot repair, bonding ductile metal onto the leading edge of a rotor blade is an undesirable technique for minimizing erosion.
One type of “field level” repair technique for improving a rotor blade's wear resistance includes applying an elastomeric material to the leading edge of the blade. Typically, the elastomeric material is applied to the leading edge as a tape. As the tape becomes worn, it can quickly and easily be removed, and a new layer of tape can be applied. Unfortunately, the elastomeric tape must be replaced more frequently than a nickel leading edge and the ability of the elastomer to resist erosion caused by the combined rain and sand is less than that of nickel. Specifically, the elastomeric tape fails to adequately absorb the impact energy of the particulate matter. Without adequate absorption capabilities, the elastomer fails to dissipate the impact energy, thereby allowing the particulate matter to erode the elastomer. Without frequent replacement of the elastomeric tape, the leading edge of the rotor blade remains unprotected.
In an attempt to supplement the deficiencies of elastomeric tape, current designs include particles disposed in the elastomer layer. The particles are mixed in with the elastomer material before it is cured onto the substrate. Unfortunately, the particles at the interface of the elastomer layer and the substrate can cause poor bonding to the substrate and/or electrochemical corrosion problems such as a galvanic coupling. Further, the embedded particles can adversely affect the elastic properties of the elastomer coating thereby reducing the ability of the elastomer coating to absorb energy from particles in the air, such as sand, water droplets and other debris.